Epoxidation process for manufacture of olefin oxide and alcohol

Abstract: An epoxidation reaction wherein an olefin and an organic hydroperoxide, preferably propylene and tertiary butyl hydroperoxide are reacted in a primary reaction zone in a liquid phase with an organic solvent, preferably tertiary butyl alcohol, in the presence of a soluble molybdenum catalyst to convert 85 to 95% of the tertiary butyl hydroperoxide and form a primary epoxidation zone reaction product; fractionating, in a primary fractionation zone, the primary zone reaction product into a primary distillate fraction comprising unreacted propylene and propylene oxide and into a primary heavy liquid fraction, comprising molybdenum catalyst, unreacted tertiary butyl hydroperoxide, tertiary butyl alcohol and side reaction products; reacting, in a secondary epoxidation reaction zone, the primary heavy liquid fraction with propylene in the liquid phase, with substantially no back mixing, for conversion of substantially all the tertiary butyl hydroperoxide and production of a secondary reaction zone product; fractionating the secondary reaction zone product into a secondary distillate fraction comprising unreacted propylene, and propylene oxide, and into a secondary heavy liquid fraction comprising molybdenum catalyst, tertiary butyl alcohol, and side reaction products.

Description: The present invention relates to the process for epoxidation of olefins with an organic hydroperoxide, in the liquid phase with an organic solvent, in the presence of a soluble epoxidation catalyst. More particularly, the present invention relates to an improved epoxidation process including: oxidation of an olefin with a hydroperoxide, typically propylene with tertiary butyl hydroperoxide, in a primary reaction zone in liquid phase with an organic solvent, typically tertiary butyl alcohol, under epoxidation conditions for conversion of about 85 to 95% of the tertiary butyl hydroperoxide and for production of a first expoxidation reaction product; fractionation of the primary epoxidation reaction product into a first distillate fraction comprising unreacted propylene and propylene oxide, and into a primary liquid fraction comprising molybdenum catalyst, unreacted tertiary butyl hydroperoxide, tertiary butyl alcohol, and side reaction products; reaction of the primary liquid fraction with propylene in a secondary epoxidation zone under epoxidation conditions for conversion of about 85 to about 95% of the tertiary butyl hydroperoxide charged in the first liquid fraction, and for production of a secondary epoxidation reaction product; fractionation of the secondary epoxidation reaction product into a second distillate fraction comprising unreacted propylene and propylene oxide and into a secondary liquid fraction comprising molybdenum catalyst, tertiary butyl alcohol, and side reaction products; and recovering propylene and propylene oxide product from the primary and secondary distillate fractions. The epoxidation reactions contemplated herein are epoxidation reactions of the type disclosed by Kollar, U.S.-A-3,351,635, as have been elaborated, for example, in Marquis, et al., U.S.-A-4,891,437, where a wide variety of olefins can be epoxidized using a wide variety of organic hydroperoxides in the presence of catalytic metallic compounds. Processes for epoxidation of C 3 -C 20 olefin hydrocarbons with an organic hydroperoxide in the presence of an epoxidation catalyst selected from compounds of molybdenum titanium, tungsten, vanadium and selenium to produce olefin oxide and an alcohol corresponding to the hydroperoxide are well known. Typically, commercial reactions employ propylene as olefin and tertiary butyl hydroperoxide as organic hydroperoxide. Products of these reactions, propylene oxide and tertiary butyl alcohol, are valuable intermediate chemicals for the manufacture of other products such as synthetic polymers and tertiary butyl ether. In GB-A-1,298,253, filed July 14, 1970, a method is disclosed for the continuous epoxidation of propylene with tertiary butyl hydroperoxide in the presence of a molybdenum catalyst, which process comprises: reacting propylene in excess with tertiary butyl hydroperoxide in the presence of tertiary butyl alcohol solvent and molybdenum catalyst in a first epoxidation zone; fractionating effluent from the first reaction zone to yield an overhead fraction containing unreacted propylene, propylene oxide and tertiary butyl alcohol, and yield a bottoms fraction containing unconverted tertiary butyl hydroperoxide, the remainder of the tertiary butyl alcohol and molybdenum catalyst; separating the bottoms fraction into a recycle stream and a purge stream and recycling the recycle stream to said first epoxidation zone; reacting the purge stream in a second epoxidation zone with propylene; and recovering the propylene oxide product from each of said epoxidation zones. The patent claims as advantages for this process that consumption of catalysts is extremely low, tertiary butyl hydroperoxide conversion is high, and at the same time selectivity (i.e., moles of propylene oxide produced per mole of tertiary butyl hydroperoxide converted) is high, thus providing a high yield of propylene oxide based upon the amount of tertiary butyl hydroperoxide charged to the process. A review of the examples shows selectivity of tertiary butyl hydroperoxide conversion to propylene oxide is 57% without the second stage reaction of the present invention and 66% with the second stage reaction. In GB - A - 1,298,253, a large recycle stream containing catalyst and side reaction products is returned to the first epoxidation zone where it is brought into contact with the epoxidation reaction mixture, including propylene oxide as well as propylene and tertiary butyl hydroperoxide reactants. This recycle stream, in addition to containing molybdenum catalyst and unconverted tertiary butyl hydroperoxide, also contains side reaction products including low molecular weight (C 1 -C 4 ) carboxylic acids. These carboxylic acids catalyze additional side reactions including propylene dimerization reactions and propylene oxide-tertiary butyl hydroperoxide esterification and etherification reaction the products of which reduce selectivity, and yield of desired product. The purge stream, which forms the charge stock to the second reactor in the disclosed process, must of necessity remove the side reaction products at a rate equivalent to the rate at which they are formed. Since the purge stream is, as disclosed in the specification, only 8 to 20% of the heavy liquid stream from the distillation zone, the recycle stream returns to the first reaction zone from 5 to 12 times the amount of side reaction products, including the low molecular weight acids, as are produced in the initial reaction. Stein, in US-A-3,849,451 issued November 19, 1974, discloses a process for a catalytic epoxidation of olefinically unsaturated compounds employing organic hydroperoxides as epoxidizing agents and employing catalysts comprised of metals such as vanadium, tungsten, molybdenum, titanium and selenium. In Stein, et al., an epoxidation reaction is carried out under autogenous pressure and at a reaction temperature sufficient to volatilize a portion of the liquid phase reaction medium, including a portion of the olefin oxide product. Preferably from 1/3 to 1/2 of the olefin oxide product and substantially none of the organic hydroperoxide is removed with the volatilized portion. Preferably, the reaction zone is partitioned into several compartments with a common overhead zone, or alternatively, several reactors in series are employed to prevent back mixing of epoxidation reaction product with reactants entering the process. In Stein, et al., the unreacted olefin and a portion (1/3 to 1/2) of the olefin oxide product are vaporized and withdrawn from each compartment or from each series reactor. Olefin recovered from the withdrawn vapor is returned as reactant to each compartment or each series reactor. Stein, et al. recognizes that the olefin oxide product enters into side reactions catalyzed by the acidic side reaction products of the epoxidation reaction. However, in the process of Stein, et al., only a portion (1/3 to 1/2) of the olefin oxide is removed from each reactor compartment or reactor in series, leaving a substantial portion (1/2 to 2/3) of the olefin oxide in contact with the acidic compounds under conditions favorable for side reactions which consume olefin oxide. Sweed, in U.S. -A- 4,455,283, issued June 19, 1984, discloses a proces, for epoxidizing olefin compounds with organic hydroperoxides in the presence of liquid solutions of dissolved molybdenum catalysts, and for recovery and recycle of the molybdenum catalyst values. In the description of the prior art, (column 2, lines 16-22) Sweed notes that distillation residue containing spent molybdenum catalyst, some alcohol, and acids as well as high boiling organic residues can be recycled directly to the epoxidation reaction zone, but direct recycle of the residue results in a build up within the system of impurities (e.g. acids) which are deleterious to subsequent epoxidation reactions. Isaacs, in U.S. - A - 4,598,057, issued July 1, 1986, discloses a process for epoxidation of olefins with organic hydroperoxides in the presence of a molybdenum catalyst with subsequent recovery of molybdenum from a spent catalyst stream derived from the epoxidation reaction product. In the detailed description of the invention at column 4, lines 57-68, Isaacs states, "As indicated above, this heavy fraction cannot be recycled directly to the epoxidation zone in view of the fact that the impurities contained therein, and most notably the acid impurities, interfere with the epoxidation reaction. The deleterious effect of these acids is particularly pronounced in a continuous system due to a build up of the concentration of these materials when a direct recycle is employed. Furthermore, partial recycle of the stream to the epoxidation reaction, over a period of time, results in accumulation of residual materials associated with the catalyst which likewise is deleterious to the overall epoxidation reaction". From the above references, it is seen that reactions for epoxidation of olefin using organic hydroperoxide in the presence of selected metal catalysts, such as molybdenum, are well known. Also, it is recognized that reactions catalyzed by acid side reaction products consume olefin oxide product and organic hydroperoxide reactant, thus reducing yield from the epoxidation reactions. Efforts, as disclosed in U.S. -A-3,849,451 and GB-A- 1,298,253 discussed above, employing more than one reactor in the epoxidation process have been made to reduce the consumption of propylene oxide and organic hydroperoxide in unwanted side reactions. These efforts have not been completely successful. Either through recycle of acidic side reaction products to the main reactor, or through incomplete removal of olefin oxide from charge to a second reactor, the olefin oxide continued to remain in contact with acidic side reaction products. These acidic side reaction products catalyze reactions such as etherification and esterification reactions, which destroy the olefin oxide product and organic hydroperoxide. Consquently, there is a requirement for improvements to the epoxidation process which will improve hydroperoxide conversion and increase product yields without increasing side reactions which consume olefin oxide and organic hydroperoxide. This invention is directed to a process where an organic hydroperoxide charge stock is reacted with an olefin charge stock having 3-20 carbon atoms in liquid phase with an organic solvent in a reaction zone in the presence of a metallic epoxidation catalyst to form a product olefin epoxide corresponding to the olefin charge stock and a product alcohol corresponding to the hydroperoxide charge, which process is improved in accordance with the present invention as follows: reacting, in a primary epoxidation reaction zone, an olefin charge stock and organic hydroperoxide in liquid phase with an organic solvent, in the presence of a metallic epoxidation catalyst, under conditions for conversion of 85-95% of the organic hydroperoxide and production of a primary epoxidation reaction product comprising unreacted olefin, olefin oxide, alcohol corresponding to the hydroperoxide, unreacted hydroperoxide, oxidation catalyst, and side reaction products including carboxylic acids; fractionating, in a primary fractionation zone, the primary epoxidation reaction product into a primary distillate fraction comprising unreacted olefin and olefin oxide, and into a primary liquid fraction comprising unreacted hydroperoxide, epoxidation catalyst, alcohol and side reaction products including carboxylic acids: reacting, in a secondary reaction zone, primary liquid fraction with excess olefin in a second epoxidation zone under conditions for conversion of 85-95% of the organic hydroperoxide present and for production of a secondary epoxidation reaction product comprising unreacted hydroperoxide, unreacted olefin, olefin oxide, alcohol corresponding to the organic hydroperoxide, epoxidation catalyst, and side reaction products; fractionating, in a secondary fractionation zone, the secondary epoxidation product into a secondary distillate fraction comprising unreacted olefin and olefin oxide, and into a secondary heavy liquid fraction comprising epoxidation catalyst, alcohol, unreacted hydroperoxide and side reaction products including carboxylic acids; and recovering unreacted olefin and olefin oxide from the primary and secondary distillate fractions. As desired, the second heavy liquid fraction may be further treated for recovery of alcohol and of the metal values from the epoxidation catalysts. In the process of the present invention, olefin reactants may be used which produce an olefin oxide which is lower boiling than, and can be separated from, the organic hydroperoxide reactant. Lower boiling C 3 -C 4 olefins such as propylene, normal butylene, and isobutylene are advantageously epoxidized in the epoxidation reactions of the present invention. Propylene, for manufacture of propylene oxide, is the preferred olefin charge for reaction in the epoxidation reactions disclosed herein. It is, however, contemplated that different olefin reactants, such as propylene and isobutylene, may be reacted separately in the primary and secondary epoxidation reaction zones. For example, propylene in the primary reaction zone and isobutylene in the secondary reaction zone. In such event, separate oxide purification zones may be required for the first and for the second reaction zones. Organic hydroperoxides which may be used in the epoxidation process of the present invention are those capable of epoxidizing the olefin reactants under the disclosed epoxidation reaction conditions such as tertiary butyl hydroperoxide and tertiary amyl hydroperoxide. Tertiary butyl hydroperoxide is preferred as reactant in the epoxidation reactions disclosed herein. The epoxidation reactions contemplated herein are carried out in the liquid phase with the olefin and hydroperoxide reactants and epoxidation catalyst dissolved in an organic solvent. The solvent may be a non reactive organic liquid in which the reactants and the catalysts are substantially soluble. Preferably, the solvent is the alcohol which corresponds to the organic hydroperoxide used as reactant in the epoxidation reaction. Where tertiary butyl hydroperoxide is the organic hydroperoxide used, then preferably the organic solvent is tertiary butyl alcohol. This is particularly convenient since common industrial processes for manufacture of tertiary butyl hydroperoxide by direct oxidation of isobutane produces a mixture of tertiary butyl hydroperoxide and tertiary butyl alcohol. By varying oxidation conditions, the ratio of tertiary butyl hydroperoxide to tertiary butyl alcohol may be varied to produce a preferred charge stock for the primary epoxidation reaction. Catalysts used in the epoxidation reactions of the present invention are preferably compounds of metals having high activity for conversion of the hydroperoxide with high selectivity to olefin oxide and alcohol. The most effective catalytic metals are vanadium, tungsten, molybdenum, titanium and selenium. The preferred metal catalyst is molybdenum, and may be present in the epoxidation reaction mixture in an amount equivalent to 20-1000 ppm based upon the weight of total reactor charge. The catalysts are normally employed as compounds soluble in the epoxidation reaction mixture. These catalysts comprise high molecular weight, highly complex compounds, which, because of their low volatility, are carried through the epoxidation reactions and subsequent fractionations as components of the non-distillate liquid fractions. Many such molybdenum compounds, having high catalytic activity, are known and have been described. For example, see Isaacs U.S. - A - 4,598,057, and also see Marquis, et al, U.S. - A - 4,626,596; 4,650,886; 4,654,427; 4,703,029; and 4,758,681, including catalysts described in the prior art discussed in these patents. The Marquis, et al patents are directed to molybdenum/alkanol and molybdenum/glycol complexes which contain high concentrations of molybdenum and are particularly useful as catalysts in the olefin epoxidation reactions. The process of the present invention may be understood more readily by reference to the drawing, which is a schematic representation of one preferred embodiment of the invention. As propylene and tertiary butyl hydroperoxide are typical preferred charge stocks to the epoxidation process of the present invention, for illustration purposes, but without intending any limitation on the scope of the invention, the drawing and the improved epoxidation process embodied therein will be described in terms of these reactants with tertiary butyl alcohol as solvent. In the drawing, a solution of tertiary butyl hydroperoxide and tertiary butyl alcohol from line 1 and molybdenum epoxidation catalyst from line 2 are mixed in line 3, and the mixture charged to primary reaction zone 4. Propylene in line 5 is charged to reaction zone 4 also. The propylene and tertiary butyl hydroperoxide are reacted in primary reaction zone 4 under epoxidation conditions, in liquid phase with the tertiary butyl alcohol solvent, in the presence of molybdenum catalyst. The epoxidation reaction in primary reaction zone 4 may be conducted under reaction conditions similar to those disclosed in Marquis, et al. U.S. - A - 4,891,437. From lines 1 and 3, the tertiary butyl hydroperoxide is charged to primary reaction zone 4 as at least a 30%, preferably a 40-75%, by weight solution in the tertiary butyl alcohol, sufficient to maintain a primary zone reaction mixture composed of more than 60 wt. % polar components (hydroperoxide, alcohol and olefin oxide). Propylene from line 5 is charged to primary reaction zone 4 in an amount sufficient to provide from 0.5 to 3.0 moles, preferably 0.9 to 2.0 moles, of charged olefin per mole of charged hydroperoxide. The epoxidation reaction and primary reaction zone 4 is conducted at a temperature in the range of 50-180°C, with a preferred range of between 90-140°C. An especially preferred range is 100-130°C. Pressure is sufficient, in the range of 1.5 - 7.0 MPa (200-1000 psig) to maintain the primary zone reaction mixture in the liquid phase. Molybdenum catalyst from line 2 is provided in an amount sufficient to maintain catalyst concentration in the primary zone reaction mixture in the range of 50-1000 ppm by weight, calculated as molybdenum, in the total reactor charge. Molybdenum concentration is preferably 200-600 ppm, and most preferably, 250-500 ppm. Advantageously, the catalyst may be a molybdenum/ethylene glycol complex as disclosed in Marquis et al US-A-4,626,596. In primary reaction zone 4, the reaction is carried out to achieve a hydroperoxide conversion of 85 to 95%, for maintaining high selectivity of converted hydroperoxide to olefin oxide and to corresponding alcohol. The use of only a small molar excess of olefin to hydroperoxide and of a more polar reaction mixture contributes to increased selectivities of the reactants. In primary reaction zone 4, the residence time of reactants may vary considerably from minutes to hours, depending upon other reaction variables such as reaction temperature, ratio of reactants, reactant concentration in the reaction mixture, catalyst concentration and catalyst activity. Generally, reaction times will run between 30 minutes and 4 hours, with 0.5 to 2.0 hours being typical. In primary reaction zone 4, the epoxidation reaction is preferably staged, with a first stage carried for a time at a selected reaction temperature and a second stage (or subsequent stages) carried out at equal or higher temperatures. For example, the epoxidation reaction may be conducted for 1 hour at 110-120°C, and for a second hour at 120-130°C. For continuous processes, a series of reactors help to achieve the objective of high selectivity to olefin oxide and alcohol of converted hydroperoxide. The use of a series of reactors in primary reaction zone 4 makes it possible to stage the addition of olefin reactant and thereby increase reaction mixture polarity and, concomitantly, decrease formation of olefin side reaction products, i.e., hydrocarbon dimers (hexenes) in the case of propylene reactant. In primary reaction zone 4, the reactor stages may be two temperature stages in the same reactor vessel, two or more back mixed reactors. such as continuously stirred tank reactors (CSTR's) in series, or, preferably, one or more back mixed reactors followed by one or more plug flow reactors (PFR's). In back mixed reactors, such as CSTR's, the concentration of reactant olefin and hydroperoxide is low, which helps retard side reactions whose rate depends on reactant concentration. On the other hand, in non-back mixed PFR reactors, the reaction products are kept away from fresh reactants, thus reducing the reactant side reactions catalyzed by acid by-products. Under conditions in epoxidation reaction zones, organic hydroperoxides (tertiary butyl hydroperoxide) are subject to thermal or catalytic decomposition leading to formation of unwanted side reaction products, including C 1 -C 4 carboxylic acids. Such decomposition and formation of side reaction products are particularly troublesome at temperatures at the range of 100-180°C, particularly above 130°C which are commonly used in epoxidation reactions of the prior art. In the drawing, the epoxidation reaction in primary reaction zone 4 is undertaken under conditions including only a small molar excess of olefin to hydroperoxide, a reaction mixture containing at least 60% polar compounds and lower reaction temperatures, preferably 100-130°C, for increasing selectivity of converted tertiary butyl hydroperoxide to formation of propylene oxide and tertiary butyl alcohol and for inhibiting propylene dimerization or reaction with tertiary butyl hydroperoxide in etherification or esterification reactions. Preferably, the epoxidation reaction in the primary epoxidation zone 4 is carried out to achieve hydroperoxide conversions range of 85-95%. Thus, 5 to 15 wt. % of the hydroperoxide will remain unconverted in the primary zone reaction mixture. The rate of hydroperoxide conversion, under epoxidation reaction conditions, is directly proportional to both hydroperoxide and olefin concentration in the reaction mixtures. In primary reaction zone 4, tertiary butyl hydroperoxide concentration and propylene concentration in the combined charge stock are quite high (approximately 20-50 wt. % hydroperoxide and 20-30 wt. % propylene within the preferred ranges). Epoxidation reactions are highly exothermic and reaction of hydroperoxide and olefin at such high initial concentrations will rapidly generate substantial heat which must be removed to control the reaction temperature within a desired range. As discussed above with reference to primary reaction zone 4, back mixed reactors may be used to mix incoming charge with existing reaction mixture, thus maintaining a moderate concentration of reactants in the reaction zone, and concomitantly maintaining a moderate reaction rate. In the drawing, primary epoxidation reaction product is withdrawn from the primary reaction zone 4 by way of discharge line 6. The primary epoxidation reaction product comprises unreacted propylene, unreacted tertiary butyl hydroperoxide, propylene oxide, tertiary butyl alcohol, molybdenum catalyst and side reaction products such as oxygenated compounds, including C 1 -C 4 carboxylic acids, and hydrocarbons containing six or more carbon atoms. Primary oxidation reaction zone product from line 6 is charged to primary distillation zone 7 where the reaction product is fractionated into a primary overhead vapor fraction comprising propylene, propane and propylene oxide, and into a primary heavy liquid fraction comprising tertiary butyl alcohol, unreacted tertiary butyl hydroperoxide, molybdenum catalyst and higher boiling side reaction products including carboxylic acids. The primary vapor fraction is withdrawn from primary distillation zone 7 via line 8 and is charged into propylene oxide distillation zone 9 as will be described herein below. In the drawing, primary liquid fraction, withdrawn from primary distillation zone 7 via line 10 comprises about 5-10 wt. % tertiary butyl hydroperoxide, 80-90 wt. % tertiary butyl alcohol, molybdenum catalyst, and 5-10% side reaction products, including carboxylic acids which catalyze side reactions which destroy olefin and hydroperoxide reactants and olefin oxide product. Primary liquid fraction from line 10 and propylene from line 12 are charged to secondary reaction zone 11 for conversion of hydroperoxide and production of olefin oxide and alcohol corresponding to the hydroperoxide. In second reaction zone 11, hydroperoxide concentration in primary liquid fraction from line 10 is low (5-10 wt. %). Consequently, steps to moderate the reaction rate, in secondary reaction zone 11 are not required. Preferably, steps to increase the reaction rate, such as use of PFR reactors and increasing ratio of propylene reactant to hydroperoxide, are employed. Reducing the residence time in secondary reaction zone 11 during which propylene and hydroperoxide reactants and propylene oxide product are in contact with the carboxylic acid side reaction products will decrease the deleterious side reactions catalyzed by these acids. In the drawing, primary liquid fraction from line 10 is highly polar, containing 5-10 wt. % tertiary butyl hydroperoxide and 80-90 wt. % tertiary butyl alcohol, with the remainder being molybdenum catalyst and side reaction products (5-10 wt. %). Thus, a substantial molar excess of propylene with respect to hydroperoxide may be charged to secondary reaction zone 11 while continuing to maintain 60 wt % or more polar compounds (hydroperoxide, alcohol, propylene oxide) in the reaction mixture. A molar ratio of propylene to hydroperoxide within the range of 5:1 to 10:1 may be charged while maintaining an overall concentration of polar compounds in the secondary reaction mixture above 60%. The more polar reaction mixture tends to stabilize the tertiary butyl hydroperoxide and render the molybdenum catalyst more active, thus improving hydroperoxide conversion and selectivity. Increased propylene concentration increases the rate of the epoxidation reaction allowing decreased residence time in secondary reaction zone 11. In the drawing, reaction temperatures in the secondary epoxidation reaction zone 11 may be in the range of 50-180°C. Lower temperatures improve selectivity of converted hydroperoxide to desired olefin oxide and alcohol products and reduce the rate of undesired side reactions, such as olefin dimerization, but also result in reduced reaction rates. Higher temperatures increase hydroperoxide conversion and olefin oxide production rates, but also increase the rates of undesirable side reactions which decrease selectivity of hydroperoxide conversion to desired products. Preferably, reaction temperatures in secondary epoxidation reaction zone 11 are maintained in the range of 110-130°C for maintaining a high rate of reaction and high selectivity of converted hydroperoxide to olefin oxide and alcohol, while controlling destructive side reactions to a low level. Pressure in secondary reaction zone 11 is maintained in the range of 1.5 - 7.0 MPa (200-1000 psig), sufficient to maintain the secondary reaction mixture in the liquid phase. In the drawing, residence time of secondary reaction mixture in secondary reaction zone 11 is preferably in the range of about 30 minutes to 2 hours, sufficient, at the selected reaction conditions, to obtain conversion of 85-95 wt. % of the hydroperoxide charge. Hydroperoxide conversion above 95% in each of the primary reaction zone 4 and secondary reaction zone 11 is neither desirable nor necessary. For example, if hydroperoxide conversion is 90% in primary reaction zone 4, then only 10% of the hydroperoxide charge enters secondary reaction zone 11. If 90% of that 10% hydroperoxide is converted in secondary reaction zone 11, then overall hydroperoxide conversion for both reaction zones is 99%. No advantage is gained by increasing hydroperoxide conversion much above 99%, while on the other hand limiting hydroperoxide conversion to 90% in the reaction zones substantially reduces the rate of deleterious side reactions. From secondary reaction zone 11 a secondary epoxidation reaction product comprising unreacted propylene, unreacted hydroperoxide, tertiary butyl alcohol, propylene oxide, side reaction products, and molybdenum catalysts is withdrawn via line 22 and charged to secondary product distillation zone 13. In secondary product distillation zone 13 the secondary reaction product is separated into a secondary distillate fraction comprising unreacted propylene, propane and propylene oxide, and a secondary liquid fraction comprising tertiary butyl alcohol, unreacted tertiary butyl hydroperoxide, molybdenum catalyst and side reaction products. The secondary liquid fraction is withdrawn from product distillation zone 13 via line 14 and is passed to further treatment (not shown) as desired for recovery or disposal of molybdenum values. Secondary distillate fraction is withdrawn from secondary distillation zone 13 via line 15 and is charged to propylene oxide distillation zone 9. As described above, primary distillate fraction from primary product distillation zone 7 is also charged via line 8 to propylene oxide distillation zone 9. In propylene oxide distillation zone 9 the primary distillate and secondary distillate are fractionated into a propylene oxide fraction, and an overhead fraction comprising propane and propylene. The propylene oxide fraction is withdrawn from propylene oxide distillation zone 9 via line 17 for further treatment (not shown) to produce a commercial propylene oxide product. Overhead fraction from propylene oxide distillation zone 9 is withdrawn via line 18 to propane splitter 19. In propane splitter 19 the overhead fraction charged is fractionally distilled into a propylene fraction and a propane fraction. The propane fraction is withdrawn from propane splitter 19 via line 20 for disposal (not shown). The propylene fraction withdrawn from propylene splitter 19 via line 21 may be recycled as propylene charge stock to primary reaction zone 4 and/or secondary reaction zone 11. EXAMPLES The process of the present invention is illustrated by the following examples which are given by way of illustration only. EXAMPLE I All experiments described herein were conducted employing bench scale reactors comprising a continuously stirred tank reactor (CSTR) and two plug flow tubular reactors (PFR).. In the experiments (A & B) illustrating the process of the present invention, the primary epoxidation reaction zone comprised the CSTR reactor followed by one PFR reactor, and the secondary epoxidation reaction zone comprised the other PFR reactor. In the comparative experiments (C & D) , embodying processes of the prior art where only a primary reaction zone is employed, the primary reaction zone comprised the CSTR reactor followed by the two PFR reactors all arranged in series. By arranging the experiments in this manner, the total reactor volume for each experiment, whether of an inventive or a comparative process, was identical with reactor volumes in the other experiments. Consequently, results obtained in the experiments may be compared for purposes of determining the advantages of the process of the present invention over the comparative processes of the prior art. Two inventive experiments, A and B, were performed. In inventive experiment A, 1.8kg (4lb) per hour of a solution comprising 55 wt% tertiary butyl hydroperoxide and 45 wt% tertiary butyl alcohol, 0.8kg (1.7lb) per hour propylene, and about 5 gm/hour of a molybdenum-ethylene glycol complex catalyst comprising about 13 wt% molybdenum were charged to the CSTR reactor. The CSTR reaction was maintained at a temperature of 120°C, and a pressure of 3.5 MPa (500 psig) sufficient to maintain the reaction mixture in the liquid phase. From the CSTR reactor the reaction mixture flowed into the first PFR reactor which was maintained at 130°C. Residence time of reactants in the CSTR and first PFR which comprised the primary reaction zone, was about 0.8 hours.. Reaction product from the first PFR reactor was withdrawn and analyzed, showing a tertiary butyl hydroperoxide conversion of 88%. This reaction product was charged to a fractional distillation column where it was separated into an overhead fraction comprising propylene and propylene oxide and a bottoms fraction comprising the tertiary butyl alcohol, unreacted tertiary butyl hydroperoxide, molybdenum catalyst and side reaction products, including carboxylic acids which catalyze further side reactions. The bottoms fraction from the primary fractionation column, containing about 7 wt % tertiary butyl hydroperoxide, was charged to the second PFR, which comprised the secondary reaction zone, at a rate of 1.6kg (3.6lb) per hour along with 0.4kg (1lb) per hour of propylene. Fractionater bottoms and propylene formed the charge to the second reactor and had an initial molar ratio of propylene to tertiary butyl hydroperoxide of about 8.5:1. In the secondary reaction zone, the reaction mixture was maintained in the liquid phase at a pressure of 3.5 MPa (500 psig) and a reaction temperature of about 130°C for a residence time of about 0.8 hours. Under such conditions, about 91.7% of tertiary butyl hydroperoxide charged to the second reactor was converted. Operating conditions for the experiment A are shown in Table I. Hydroperoxide conversion and selectivities to production of propylene oxide and tertiary butyl alcohol are shown in Table II. Example B, which also embodied the process of the present invention, was operated under substantially the same conditions as experiment A, except reaction temperature of the primary reaction zone was increased to 130°C. Operating conditions for this experiment B are shown in Table I and conversion and yield for the reaction are shown in Table II. Experiment C and D are comparative examples employing a process of the prior art. In these experiments only a primary reaction zone was used. This primary reaction zone comprised the CSTR reactor and the two PFR reactors employed in the inventive experiments A & B described above. However, in these comparative experiments, the CSTR reactor and the two PFR reactors were arranged in series in a single primary reaction zone. In operation, about 1.8kg (4lb) per hour of the solution comprising about 55 wt% tertiary butyl hydroperoxide and 45 wt% tertiary butyl alcohol was charged along with 0.8kg (1.75lb) per hour of propylene and about 5 g/hr of molybdenum-ethylene glycol complex catalyst to the CSTR reactor. The CSTR was maintained at a temperature of about 120°C and a pressure of 3.5 MPa (500 psig) sufficient to maintain the reaction mixture in the liquid phase. Reaction mixture from the CSTR reactor flowed into the first PFR reactor which was maintained at 130°C and from there into the second PFR reactor which was likewise maintained at 130°C. Residence time in the primary reaction zone for this comparative experiment was about 1.6 hours. Reaction product from the last PFR in the primary reaction zone was withdrawn and analyzed showing a tertiary butyl hydroperoxide conversion of 97.3%. Operating conditions for experiment C are shown in Table I, and conversion and yields from the reaction are shown in Table II. Experiment D was substantially a duplicate of example C. Operating conditions for D are shown in Table I and conversion and yields are shown in Table II. Examining the results for both inventive and comparative experiments reported in Table II, it can be seen that the process of the present invention (experiments A & B) clearly exhibits advantages over the comparative process (experiments D & C) of the prior art. That is, in the inventive experiments A and B the overall tertiary butyl hydroperoxide conversion is 99% while the tertiary butyl hydroperoxide conversion for the two comparative experiments D & C are 97.3% and 98.4% respectively. Thus, tertiary butyl hydroperoxide conversion is somewhat improved and selectivity of converted tertiary butyl hydroperoxide to desired propylene oxide and tertiary butyl alcohol products is substantially improved in the process of the present invention as compared to the process of the prior art. For example, comparing inventive experiment A with comparative experiments C and D, the yield of propylene oxide per mole of tertiary butyl hydroperoxide is 0.93:1 compared to 0.88:1. This is 5% improvement in propylene oxide yield. Similarly, the yield of tertiary butyl alcohol per mole of tertiary butyl hydroperoxide is 0.95:1 compared to 0.91:1, for a 4% improvement in yield.

Claims: An epoxidation process wherein olefin and an organic hydroperoxide are reacted in liquid phase with a polar organic solvent in the presence of a soluble epoxidation catalyst for production of olefin oxide and alcohol corresponding to the hydroperoxide, which comprises: a) charging hydroperoxide, organic solvent and a first olefin to a primary epoxidation reaction zone under epoxidation reaction conditions for conversion of 85-95% of the hydroperoxide and production of a primary reaction zone product comprising first olefin oxide, unreacted first olefin, unreacted hydroperoxide, alcohol corresponding to hydroperoxide, organic solvent, epoxidation catalyst and side reaction products; b) fractionating, in a primary fractionation zone, primary reaction zone product into a primary distillate fraction comprising unreacted first olefin and first olefin oxide, and into a primary liquid fraction comprising alcohol, solvent, unreacted hydroperoxide, epoxidation catalyst and side reaction products; and c) charging primary liquid fraction and a second olefin to a secondary epoxidation reaction zone under epoxidation reaction conditions for conversion of 85-95% of organic hydroperoxide in the liquid fraction and production of a secondary reaction zone product comprising second olefin oxide, unreacted olefin, unreacted hydroperoxide, alcohol, solvent, epoxidation catalyst and side reaction products. A process as claimed in Claim 1, wherein the first and second olefins are propylene, n-butylene or isobutylene, wherein the organic hydroperoxide is tertiary butyl hydroperoxide or tertiary amyl hydroperoxide, and wherein the epoxidation catalyst is a molybdenum compound soluble in the primary reaction zone product and in the secondary reaction zone product. A process as claimed in Claim 1 or Claim 2 wherein epoxidation reaction conditions in the primary reaction zone include, a weight ratio of hydroperoxide to polar solvent of 30:70 to 75:25 in the charge, a molar ratio of first olefin to hydroperoxide of 0.9:1 to 3:1 in the charge, reaction temperature in the range of about 100-140°C, pressure in the range of 1.5 to 7.0 MPa (200-1000 psig) to maintain the reaction mixture in the liquid phase, and residence time in the range of 30 minutes to 2 hours sufficient for conversion of about 85 to about 95% of the hydroperoxide charge, and wherein epoxidation reaction conditions in the secondary reaction zone include a molar ratio of olefin to hydroperoxide of 5:1 to 10:1 in the charge, reaction temperature in the range of 100-140°C, pressure in the range of 1.5 to 7.0 MPa (200-1000 psig) to maintain the reaction mixture in the liquid phase, and residence time in the range of 30 minutes to 2 hours sufficient for conversion of about 85 to about 95% of the hydroperoxide in the first liquid fraction charge. A process as claimed in any one of Claims 1 to 3 wherein the polar solvent is the alcohol corresponding to the hydroperoxide, and wherein the first olefin and the second olefin are the same compound. A process as claimed in any one of Claims 1 to 4 wherein propylene and tertiary butyl hydroperoxide are reacted in liquid phase with tertiary butyl alcohol solvent in the presence of a soluble molybdenum catalyst, for production of olefin oxide product and alcohol product. A process as claimed in Claim 5 wherein in the primary reaction zone the hydroperoxide and alcohol charge weight ratio is from 30:70 to 75:25, the molar ratio of propylene to hydroperoxide is from 0.9:1 to 3.0:1 and molybdenum is present at from 50 to 1000 ppm based on total reactor charge. A process as claimed in Claim 5 or Claim 6 wherein the molar ratio of propylene to hydroperoxide in the secondary reaction zone is from 5:1 to 10:1. A process as claimed in any one of Claims 5 to 7 wherein the primary reaction zone temperature is maintained in the range of 100-140°C, the weight ratio of tertiary butyl hydroperoxide to tertiary butyl alcohol is in the range of 45:55 to 75:25; and the molar ratio of propylene to hydroperoxide is in the range of 0.9:1 to 2.0:1. A process as claimed in Claim 8 where the secondary reaction zone temperature is maintained in the range of about 100-140°C. A process as claimed in Claim 8 wherein the temperature in the primary reaction zone is staged, with a range of 100-120°C during a first reaction period and a range of 120-130°C in a second reaction period.

Inventors: KEATING KENNETH PATRICK, MARQUIS EDWARD THOMAS, MUELLER MARK ALLAN, KEATING, KENNETH PATRICK, MARQUIS, EDWARD THOMAS, MUELLER, MARK ALLAN
Applicants: TEXACO CHEM INC, TEXACO CHEMICAL INC.
Application year: 1994
Main classification: C07D301
IPC codes: C07D303, B01J31, C07D301, C07C29, C07B61